How to Size a Sand Screw for Optimum Fines Retention

April 14, 2021
Tips and tricks for sizing a Fine Material Screw Washer.

Sand Screws, also known as Fine Material Screw Washers, have been around for a long time. They provide the primary functions of washing, classifying and dewatering of sand and other fine solids. These machines are easy to operate and require minimal horsepower or kilowatts per ton per hour of solids feed in comparison to other sand washing and dewatering processes. They can be used by themselves or as part of a larger wet processing plant. In either case, a sand screw must be sized correctly to provide optimum fines retention.

Before getting into how to size a sand screw for an application, it is important to know how these machines work.  

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How Do Sand Screws Work?

Sand or fine material enters through the feed box on a baffle plate. The heavier solids sink to the bottom of the box, while the finer fraction floats to the surface and flows over the weirs. The material that settles is then conveyed from the pool area up a sloped washer box to the discharge end.

As the solids are conveyed beyond the pool area, they enter the dry deck section of the washer box, at which point the water begins to separate from the material. An extra-long dry deck area provides maximum dewatering of the fine solids. The curvature of the rotating screw flights conforms to the curved section of the washer box to efficiently convey, wash (through the tumbling of the sand particles) and dewater. A channel is provided opposite the conveying side to allow water to drain from the material and return to the pool area. 

Sizing a Sand Screw

The sizing and selection of a Fine Material Screw Washer is based on the type of feed solids, feed gradation, desired product specification, capacity required and water volume if in a feed slurry.

All these factors must be taken into consideration when properly selecting the size of a sand screw. Failure to review all these parameters could result in improper size selection. 

Data Required to Make Size Selection

Sand screws must be sized for both water and solids. The percent passing 50 mesh (297 µm) in the product dictates the screw speed, while the amount of minus 200 mesh (74 µm) material dictates the water volume required for efficient operation.

In order to size a sand screw, you need to know:

  • The tons per hour of solids feed
  • The particle size distribution gradation of the sand solids
  • Particle size specification requirements for the sand product or products
  • Water volume (if any) associated with the sand solids

From this data, two charts are used to size a Fine Material Screw Washer. The first chart shows the coarse sand tph capacities and water volumes that standard sizes of McLanahan Fine Material Screw Washers can handle while saving fine sand solids retained on 100 mesh or 0.150 mm to 200 mesh or 0.075 mm.

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The water volumes listed are capacities allowing for most sand to be retained on the sieves listed.

Capacities are based on:

  • 3/8” or 10 mm x 0
  • Having a specific gravity of 2.6 and higher
  • Weighing 90-100 pounds per cubic foot (1.45-1.6 mt/m3) or more

The water volumes listed are the maximum allowable US gallons per minute when maintaining a 1.03 specific gravity.

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The second chart shows the recommended screw shaft speed based on the percent passing 50 mesh or 300 micron in the anticipated product after fines have been removed. As the chart shows, the screw shaft speed needs to be reduced to dewater finer sands.

When screw shaft speed is reduced, the unit’s capacity is lowered the same percent as the speed. For example, if the speed is reduced by 50%, the capacity of the unit will drop by 50%. If the speed is not lowered, fine sand can fill up in the washer box tub, causing fine sand to overflow and be lost.

If the screw washer is running too fast, the solids do not have a chance to stay on the flight long enough to dewater. The material will fall off the flight and return to the pool area. Eventually, the box will fill up with sand and overflow with the ultra-fines, possibly stalling the screw shaft.

What about Water in the Feed?

Water that comes in with the feed must also be taken into account. A washer box can only process so much water while retaining materials to a certain size. Using the provided chart, you can determine the size of a washer box that must be used in order to save to the desired mesh product.

If too small of a washer box is used, desired fine fractions that may need to be saved will overflow the weirs. In some cases, a larger screw washer must be selected regardless of the sand-required capacity.

As an example, if the project calls for a screw washer to handles 175 tph but the screw has 800 US gallons per minute coming in with the feed and the producer needs to save down to 200 mesh, then a 66”x36’ Fine Material Single Screw Washer must be used. If a 44”x33’ Fine Material Single Screw Washer would have been used based on capacity alone, then the operator would have only saved down to approximately 180 mesh.

Fine Material Screw Washer must maintain a calm pool area for efficient classification.

In order to properly retain solids in a screw washer, a calm pool area must be maintained. The feed box located within the baffle plate is used to direct material away from the calm pool area. If the pool area is turbulent, finer solids that may be desirable in the product will overflow the weirs.

The baffle plate helps to separate the washer box feed area and keeps the pool area calm. In some applications, it may be desirable to remove the baffle plate and feed the material close to the back plate by the weir. This will help agitate the pool area and overflow the material the producer wishes to remove. 

Sizing Screw Washers Rule of Thumb

It is important to review the amount of material passing 200 mesh (75 µm) fines in the feed. Too little water and the operator will not be able to remove these ultra-fines.

As a general rule of thumb, a Fine Material Screw Washer requires 50 US gallons per minute of water to remove 1 stph of minus 200 mesh (75 µm) fines in the feed solids. 

High Water Volume Screws

Because screws need to be sized for both water and solids, feeds with large water volumes and low solids volume can end up being sized for the water and not the solids. This can result in a machine that is sized correctly for the amount of water but may be too large to provide optimum fines retention. 

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That’s why High Water Volume Fine Material Screw Washers were invented: to provide applications with low solids volume and high water volume in the feed with a properly sized solution for washing, classifying and dewatering sand and other fine solids. High Water Volume Fine Material Screw Washers allow smaller diameter screw shafts in a larger washer box. For example, a 44” screw shaft may be placed in a washer box meant for a 66” screw. They can save the producer in capital cost expenditures by buying a correctly sized machine for their specific application instead of one that is too large and will take longer to see an ROI.

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When sizing a sand screw, you need to consider more than just the tons per hour. A Fine Material Screw Washer needs to be sized for the tons per hour of sand, the gradation of the anticipated sand product and the water volume associated with the sand solids. Considering all this data upfront will allow you to select the right Fine Material Screw Washer for optimum fines retention. 

Tags: Mining, Washing and Classifying, Aggregates, Minerals, Dewatering